1. Field of the Invention
The present invention relates to a heating apparatus applicable to an image forming apparatus, such as a copying machine, a laser beam printer, or a facsimile machine.
2. Description of the Related Art
In general, an image forming apparatus includes a heating device maintained at a predetermined temperature to heat and fix an image formed on a recording material together with a pressing roller that can be pressed against the heating device. The image forming apparatus conveys each recording material to a nip portion and sandwiches the recording material between the heating device and the pressing roller. At the nip portion, an image formed on the recording material is heated and fixed to the recording material. For example, when the heating device is a film heating type, a heater including a resistance heating member formed on a ceramic substrate is provided inside a cylindrical film.
The above-described resistance heating member may be employed in a heater to be used in a region where an available voltage of a commercial AC power source is a 100V type (e.g., in a voltage range from 100 V to 127 V) as well as in a heater to be used in a region where the available voltage is a 200V type (e.g., in a voltage range from 200 V to 240 V). In this case, if the resistance value of the heaters is the same, a serious rise occurs in harmonic current and flicker because electric power supplied to the heater is proportional to the square of the applied voltage.
The maximum electric power that can be supplied to the heater in the region where the available voltage of the commercial AC power source is 200 V is four times the maximum electric power that can be supplied to the heater in the region where the available voltage of the commercial AC power source is 100 V. If the maximum electric power that can be supplied to the heater becomes greater, the harmonic current and flicker that occur in the electric power control of the heater become larger.
Accordingly, it is required to differentiate the resistance value of a heater to be used in the region where the available voltage of the commercial AC power source is 100 V from the resistance value of a heater to be used in the region where the available voltage of the commercial AC power source is 200 V.
Further, a relay switch can be used to switch the heater resistance value, as conventionally known as a method for universalizing a device for both the region where the available voltage of the commercial AC power source is 100 V and the region where the available voltage of the commercial AC power source is 100 V.
For example, as discussed in Japanese Patent Application Laid-Open No. 7-199702 and U.S. Pat. No. 5,229,577, there are conventional apparatuses that employ a method for switching the resistance value of a heater according to the voltage of the commercial AC power source.
More specifically, the above-discussed apparatus includes a first conductive path and a second conductive path extending in a longitudinal direction of the heater. The above-discussed apparatus can perform switching between a first operational state where the first conductive path is connected in series to the second conductive path and a second operational state where the first conductive path is connected in parallel to the second conductive path.
According to the method discussed in the above-described Japanese Patent Application Laid-Open No. 7-199702, a make contact (always open contact) or break contact (always closed contact) relay and a break-before-make contact (BBM contact) relay are used to switch a connection pattern of two conductive paths between “series” and “parallel.” In this case, the above-described BBM contact relay can be replaced by two make contact relays or a combination of a make contact relay and a break contact relay. On the other hand, two BBM contact relays are used in the switching method discussed in the above-described U.S. Pat. No. 5,229,577.
According to the above-described conventional methods, the resistance value of the heater can be switched by determining whether the power source voltage is the 100V type or the 200V type and changing the connection pattern of the heater conductive paths between “series” and “parallel”, without changing the heat generation area of the heater.
However, according to the above-described methods, if a power source voltage detection unit or a heater resistance value switching relay fails, over-power may be supplied to the heater. For example, in a situation where the voltage is supplied from a 200 V power source, if the heater operation goes into a state where the resistance value becomes smaller, the electric power supplied to the heater possibly increases to four times the normal value and the heater may be immediately broken.
A conventional failure detection circuit that relies on a temperature detection element, such as a thermistor, a temperature fuse, or a thermo SW, requires a relatively long time to convert a detected voltage value to a temperature value. Therefore, the response speed in detection is insufficient and the detection can be delayed significantly.
Therefore, in a heating apparatus that is configured to switch the resistance value of a heater, it is required to surely detect, at early timing, a failure state where over-power is supplied to the heater. Further, even in a state where a bidirectional thyristor (which may be referred to as “TRIAC”) is employed to control an operational state of the heater or electric power supplied to the heater, it is required to employ a method capable of surely and promptly detecting the failure state where over-power is supplied to the heater.